This invention relates to a bevel gear for use in power transmission devices in a variety of machinery, and more particularly, to a bevel gear suited for use in automotive differential gearing.
Straight bevel gears and spiral bevel gears are generally manufactured by preparing a gear material 100 as shown in FIG. 1A by forging or machining, and then forming a tooth 101 in the material 100 by cutting as shown in FIG. 1B, resulting in a bevel gear 102 as shown in FIG. 1C. These bevel gears can also be produced by electric discharge machining (not shown) as disclosed in Japanese Patent Laid-Open Publication No. HEI-9-323219. The tooth 101 of the bevel gear 102 is shaped to gradually increase in dimensions from a small-diameter end to a large-diameter end. However, a fillet curve between a tooth flank A of the tooth 101 and a bottom land B has a radius of curvature R being identical from the small-diameter end to the large-diameter end (See cross-sectional views of FIG. 1D and FIG. 1E, illustrating the small-diameter end and the large-diameter end, respectively). A radius of curvature R of the fillet curve depends on a radius of curvature R at the small-diameter end for providing good meshing with a mating gear.
This kind of related bevel gear faces, however, a problem described below when used in automotive differential gearing. That is, the bevel gear used in automotive differential gearing is adjusted to contact at its teeth a mating gear at a position close to the large-diameter end with a larger tooth width, so as to deal with the power transmission load. This causes the gear to be likely to break at the dedendum of the large-diameter end because of the small radius of curvature of the fillet curve depending on that of the small-diameter end. To prevent such breakage, it is necessary to disadvantageously use a large-sized bevel gear for obtaining required strength. This has been a problem to be solved.
This invention was made with the above problem in mind, and has an object of providing a bevel gear having sufficient strength from a small-diameter end to a large-diameter end.
In order to achieve the above object, a bevel gear according to this invention has a radius of curvature of a fillet curve between a tooth flank and a bottom land at a large-diameter end made greater than that at a small-diameter end.
According to this invention, a radius of curvature at a large-diameter end is made greater than that at a small-diameter end, increasing the strength of the large-diameter end which is otherwise likely to be broken, and thereby achieving sufficient, even strength from the small-diameter end to the large-diameter end. This enables reduction in size of the bevel gear and thus the reduction in size of a transmission device including the bevel gear, leading to reduction in cost thereof.